I. Field of the Invention
The present invention relates to wireless telecommunications. More particularly, the present invention relates to a novel and improved method and apparatus for conducting telephone calls or other communications between wireless telecommunications systems that supports inter-system soft handoff.
II. Description of the Related Art
FIG. 1 is a diagram of a cellular telephone system configured in accordance with the use of code division multiple access (CDMA) radio frequency (RF) signal processing techniques. To conduct a telephone call or other communication, a subscriber unit 18 interfaces with one or more base stations 12 using CDMA modulated radio frequency signals over which both traffic and signaling data are exchanged. Each interface includes a forward link traffic channel transmitted within a forward link RF signal from the base station 12, and a reverse link traffic channel transmitted within a reverse link RF signal from the subscriber unit 18. Using the data exchanged, a subscriber unit 18 communicates with numerous other types of systems by way of base station controllers (BSC) 10 including public switched telephone network (PSTN) 19 and any systems coupled thereto. Typically, BSC's 10 are coupled to PSTN 19 and to base stations 12 via wire line links including, for example, T1 or E1 links, the use of which is well known in the art.
The IS-95 CDMA over the air interface standard (IS-95) promulgated by the Telecommunications Industry Association (TIA) is a set of standards for providing wireless telephone service via CDMA modulated RF signals. In accordance with the IS-95 standard, the forward and reverse link traffic channels are created by direct sequence modulating the data being transmitted with forward and reverse link traffic channel codes known to both the subscriber unit 18 and base station 12. Additionally, each base station 12 transmits a pilot channel via the periodic modulation of pilot data with a predetermined pilot channel code, with the offset associated with the pilot channel generated by each base station 12 being set to one of five hundred and twelve (512) time offsets to allow for identification of each base station 12. A subscriber unit 18 may detect the presence of the base station 12 by searching for the associated pilot channel. The use of channel codes for forming channels allows multiple communications to be conducted within a single RF bandwidth, and also allows adjacent base stations 12 to conduct communications using the same RF bandwidth. Both these characteristics allow for more efficient use of the available RF bandwidth, which is one of the advantages of CDMA technology.
The IS-95 standard also calls for data to be transmitted between base stations 12 and subscriber unit 18 in 20 milliseconds (ms) frames. To facilitate the even distribution of all the data being transmitted, the transmission time of each frame is set to one of sixteen frame offsets which occur some multiple of 1.25 ms after a frame edge. The frame offset is the amount of time the subscriber unit 18 delays transmission of a frame after a frame edge. The frame edge timing is provided to each subscriber unit 18 via synchronization information transmitted to each subscriber unit 18 using a synchronization channel transmitted within the forward link signal, and the frame offset for a particular communication is determined at the beginning of each communication which will remain the same throughout the call.
In accordance with the IS-95 standard, data is transmitted at one of four rates during each 20 ms frame, with the four rates being referred to as full rate, half rate, quarter rate and eighth rate. Cellular networks operating at full rates of either 9.6 kilo-bits per second or 14.4 kilo-bits per second are known, with the lower rates for each configuration being approximately equal to the next higher rate divided by a factor of two. The four data rates associated with a full rate of 9.6 kilo-bits per second are referred to as Rate Set One, and the four data rates associated with a full rate of 14.4 kilo-bits per second are referred to as Rate Set Two. To facilitate understanding of the exemplary embodiment of the invention described below, the number of bits transmitted at each frame rate for Rate Set One and Rate Set Two are shown in Tables I and II respectively, along with their associated probability of transmission during a typical voice conversation, however, no particular rate set is necessary to practice the invention, although the rate sets described are preferred because they comply with known standards.
TABLE I ______________________________________ 9.6 Kbps Rate Set Frames Frame Type Frame Length (Bits) Probability ______________________________________ Full Rate 256 0.291 Half Rate 160 0.029 Fourth Rate 120 0.072 Eighth Rate 96 0.598 ______________________________________
TABLE II ______________________________________ 14.4 Kbps Rate Set Frames Frame Type Frame Length (Bits) Probability ______________________________________ Full Rate 352 0.291 Half Rate 208 0.029 Fourth Rate 136 0.072 Eighth Rate 104 0.598 ______________________________________
Still referring to FIG. 1, sections of three CDMA cellular systems (which are also referred to as CDMA Cellular Land Networks--CCLN) are shown, each of which is comprised of a BSC 10 and the set of base stations 12 to which it is coupled. Each base station 12 interfaces with a set of the subscriber units 18 located within the corresponding coverage area 15. Most coverage areas 15 are divided into sectors 17 which correspond to separate antenna and RF signal processing systems within each base station 12 that are normally configured in a direction specific manner. Coverage area 15A is shown as a single sector 17 which generally corresponds to one or more omnidirectional antennas and RF signal processing systems. Billing, subscription and call routing functionality are typically provided within BSC's 10, or via the use of other systems coupled to BSC's 10 which are not shown. Separate CDMA cellular system's are generally used either by different service providers, or by the same service provider when the number of base stations 12 necessary to service an area exceeds the capacity of a single BSC 10.
As shown in FIG. 1 subscriber unit 18C is interfacing with a single base station 12 while subscriber unit 18A is interfacing with multiple base stations 12. The state of interfacing with multiple base stations 12 at the same time is referred to as soft handoff. Soft handoff can be contrasted with hard handoff during which a subscriber unit drops a first interface with a base station before establishing a second interface with another base station. Soft handoff increases the diversity of signal source for a corresponding subscriber unit since multiple connections are maintained at all times during the communication. Because of this increased diversity of signal source, soft handoff is typically considered superior to hard handoff. Hard handoff is nonetheless performed in most non-CDMA wireless cellular systems because each adjacent base station employs a different range of RF frequencies to conduct telephone calls, and most subscriber units can only operate at a single frequency band at any given time.
To perform soft handoff within a CDMA cellular system in accordance with the IS-95 standard, various functions must be performed by each BSC 10. These functions include selecting a best instance of reverse link data from a set of instances of reverse link data received (data-selection), as well as generating multiple instances of forward link data for transmission to the subscriber unit 18 (data-broadcasting). The multiple instances of the reverse link data are generated during a soft handoff by the set of base stations 12 with which the subscriber unit 18 has established an interface. Conversely, each instance of the forward link data generated is for transmission to transition of the same set of base stations 12. A description of the procedures associated with one method for performing a soft handoff is set forth in U.S. Pat. No. 5,267,261 entitled "MOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM" assigned to the assignee of the present invention and incorporated herein by reference. Additionally, each BSC 10 performs various power control operations necessary to the proper operation of a CDMA cellular telephone system as set forth in the IS-95 standard and a description of one implementation is provided in U.S. Pat. No. 5,056,109 entitled "METHED AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM" assigned to the assignee of the present invention and incorporated herein by reference.
Another feature employed in CDMA cellular telephone systems is softer handoff. Softer handoff, is the state of simultaneously interfacing with two sectors 17 of the same base station 12, as is illustrated by subscriber unit 18B. In general, the multiple instances of the reverse link RF signal received from a subscriber unit 18B in softer handoff are combined within the associated base station 12, and the resulting single set of reverse link data is transmitted to the associated BSC 10. Similarly, any duplication of forward link data to be transmitted from each sector to the subscriber unit 18 in softer handoff is performed within the associated base station 12.
As shown in FIG. 1, subscriber unit 18C is located between the coverage areas 15 of two base stations 12 that are coupled to different BSC's 10, and therefore at the edge of two CDMA cellular systems. To provide complete mobility within a metropolitan region it is desirable to allow subscriber unit 18C to transitions into the second CDMA cellular system while continuing the call, as many metropolitan areas and other regions are large enough to require multiple CDMA cellular systems for complete coverage. While the BSC 10 processing the call could be switched as subscriber unit 18C moved from the first CDMA cellular system to the second CDMA cellular system, switching BSC's 10 would preclude the use of conventional soft handoff techniques as no single BSC 10 would have access to all the information generated by the first and second base station that would be necessary to perform data-selection and data-broadcast during the transition.
Without data-selection and data-broadcast, soft handoff cannot be performed in the normal manner. While a hard handoff similar to that performed in non-CDMA cellular telephone systems may be employed, during which the BSC 10 processing the call is switched, hard handoff is less desirable than soft handoff as noted above, and performing hard handoff within a CDMA cellular telephone system is especially difficult because use of same frequency in adjacent base stations creates interference not present in the non-CDMA systems. Therefore, a system and method for allowing a call to be processed across CDMA cellular systems that supports soft handoff across CDMA cellular system borders is highly desirable. The present invention is directed to such a system and method.